Cellular differentiation involves mechanisms to establish and maintain gene expression patterns, whether they are states of activity or silence. The factors required to establish the initial pattern differ in many cases from the factors that promote transcriptional memory. One group of proteins, the Polycomb group (PcG) complex, have emerged as critical regulators in the maintenance of transcriptional repression. Polycomb-mediated repression is hypothesized to involve a repressive chromatin structure. It remains unclear how PcG complexes are recruited to mammalian gene targets and how the higher order structure of the target gene contributes to the transcriptional repression. A genetically weIl-characterized target of the mammalian PcG complex is the Ink4a tumor suppressor locus. Repression of the Jnk4a locus by 8mi-1, a polycomb gene, has lead to tumorigenesis in the hematopoietic lineage. I plan to study the mechanism of polycomb group recruitment to the hk4a locus. Specifically, I will characterize the higher order chromatin structure of the Ink4a locus when repressed by Polycomb complexes via cell culture approaches, and I will use in vitro approaches to characterize key aspects of function of PcG proteins on this locus.